1. Field of the Invention
The present invention relates generally to digital communication networks. In particular, the present invention is directed toward a resilient interface that connects two frame relay communication networks.
2. Related Art
A data communications network serves to transport digital information between a number of locations. A typical data communications network consists of various physical machines called "nodes", interconnected by conduits called "links." Some of the network nodes have links to the network user's devices.
Circuit switching and frame-based switching are two fundamentally different transport technologies used to build data communication networks. Circuit switching is a technology that builds a physical data path dedicated to a set of users. An example of this is the Public Switched Telephone Network. Once a call is established, the network transmission resources associated with the path are dedicated to the call, whether they are used or not. Frame-based switching is a general term for technologies based on the concept of shared network transmission resources. User data is divided into various data units called frames, packets, datagrams, or cells, depending on the specific technology employed. The data unit contains routing or connection information, used by nodes along the path to route or switch the data unit to the link connected to the next node on the way to the eventual destination. As a result, data units destined for different termination points typically share physical links between intermediate nodes in the path. Transmission resources associated with these links are more fully utilized because the statistical distribution of periods of activity and inactivity of many users results in less overall inactivity of the shared resources.
The connections between users of the Frame Relay network are called Virtual Circuits because they are logical constructs of the network, and not dedicated physical circuits. Virtual circuits can be categorized as permanent or switched. A Permanent Virtual Circuit (PVC) is a relatively long-duration logical path between two Customer Premises Equipment (CPE) devices, configured by the network administration, and is typically not dismantled after a specific communications session. A Switched Virtual Circuit (SVC) is a relatively short-duration logical connection set up in response to a call made by the CPE, and is typically disconnected after a specific communications session.
Frame Relay (FR) is technically a data network access standard defined by Frame Relay Forum Implementation Agreements and ITU-T recommendation I.122. It defines a User-to-Network Interface (UNI), a framing protocol for the link connecting Customer Premises Equipment to the Frame Relay network. The frames typically have a 3-byte header, 3-byte trailer, and a variable payload of up to 4096 bytes in length. The header includes a 1-byte start flag, and an address field containing a Data Link Connection Identifier (DLCI) that identifies unique virtual circuits on the UNI. The frame relay trailer consists of a two-byte frame check sequence field, and a 1-byte end flag field. A Network-to-Network Interface (NNI), a minor variation of the UNI specification, is also specified for connections between separate Frame Relay networks.
The frame-based transport technology employed in the interior of the frame relay network is not visible to the user, and is typically proprietary to the providers of specific frame relay switching equipment. The links on the interior of the frame relay network are referred to as trunks to differentiate them from UNIs and NNIs. Whereas UNIs are standardized access links between CPE and a frame relay network, Network-to-Network Interfaces (NNIs) provide a standardized method of inter-connecting autonomous frame relay networks which use incompatible interior trunking protocols. This inter-network transparency is increasingly important, as de-regulation of the telecommunications industry results in merged and partnered companies with merged and partnered frame relay networks.
A key advantage of Frame Relay networks is that the data units can be dynamically routed around specific points of congestion or failure within the network. This self-healing property is compromised when an NNI link connects two Frame Relay networks, because the NNI constitutes a single point of failure. This means that while the individual networks can route virtual circuits around failures of interior trunk links, a failure of an NNI link is disruptive until the failure is detected, diagnosed, and repaired. It would be highly preferable if the virtual circuits could also be routed around the failure of an NNI link.